Integrated Cascading Plant

ABSTRACT

An integrated refrigeration and air conditioning plant working with environmentally friendly natural refrigerant carbon dioxide (CO2) and an ozone friendly non CO2 refrigerant. The low temperature refrigeration is provided by evaporating CO2 and the fluid is used as a secondary refrigerant, predominantly in liquid mode, for medium temperature refrigeration. The heat rejection from CO2 occurs to another upper stage cascaded refrigeration system that operates with a ozone friendly non CO2 refrigerant. Since air conditioning applications require cooling at temperatures higher than refrigeration for food sector, cooling for this range is derived from the non CO2 refrigerant.

FIELD OF INVENTION

The invention relates to refrigeration and air conditioning systems, including such systems when used in a supermarket setting. It specifically addresses the lacuna in integrating seemingly different, but intricately related, domains of cooling requirements at various temperature levels. The objectives are sought to be achieved in a manner that is environmentally friendly, energy efficient and operated in an intelligent way that marries the thermodynamic, fluid dynamic interface at two different refrigerants.

BACKGROUND TO THE PRESENT INVENTION

In this document the numbering of refrigerants is in accordance with the protocol recommended by the learned societies relevant to the profession of refrigeration and air conditioning, such as American Society of Refrigeration, Heating and Air conditioning Engineers (ASHRAE), International Institute of Refrigeration (IIR), Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH), Japan Society of Refrigerating and Air Conditioning Engineers (JSRAE).

Whilst the following discussion is in terms of a refrigeration and air conditioning systems in supermarkets, a person skilled in the art will appreciate that the invention may be applied for non supermarket applications.

The commitment of major supermarket owners to reduction of greenhouse gas emissions makes it imminent that they use “green” cooling technologies. Refrigeration contributes to about 70% of their energy consumption which is responsible for indirect emissions and virtually 100% contribution towards direct emissions resulting from leakages of refrigerants from the cooling systems.

The current Australian refrigeration and air conditioning trade practices are unable to reduce the leakages below 10% of initial charge per year. A supermarket has anywhere between 600-1500 kg refrigerant on site leading to at least 60-150 kg of gas leaking out each year. While this may not appear much, the amount of supermarkets Australia wide is 1500 and therefore the collective damage caused by their leveraging emissions is high.

HFC 134a, the most popular air conditioning refrigerant has a global warming potential of 1300 times that of CO2 and the low temperature refrigerant R404a has 3300 for this value. It is logical to substitute these where possible with CO2 whose global warming potential is just 1. However, CO2 has not made much of market penetration due to (i) requirements of high pressures in operating systems and (ii) problems associated with its storage in liquid form. Consequently, this refrigerant has primarily been used as a low temperature refrigerant and therefore not across the range of refrigerant temperatures required by supermarkets.

Current state-of-the-art uses CO2 in the direct expansion mode for low temperature (˜−30 ° C.) refrigeration, CO2 in liquid recirculation mode for medium temperature (˜−10 ° C.) refrigeration. The air conditioning plant is independent of the refrigeration plant for the above reasons. However, there has been one instance of use of liquid CO2 recirculation for air conditioning applications which entailed the direct heat exchange between CO2 and the air to be circulated for cooling the supermarket. This gives rise to a significant safety hazard (suffocation) if there is any significant leakage of CO2 into the circulating air. Condensing/cooling of CO2 is obtained, predominantly, by the use of R507a/404a, although 134a has been used in the recent years.

OTHER DRAW BACKS OF THE PRESENT STATE-OF-THE-ART

In order to limit the operating pressures, the cascading medium temperature in conventional refrigeration systems is about −10° C. The upper stage cascading with a synthetic refrigerant mandates cooling to be obtained at a maximum of −15° C. At this temperature the specific volumes of refrigerants are so large that either large reciprocating compressors need to be used or screw compressors have to be chosen, both options being economically unattractive.

As foreshadowed above, use of CO2 circulation in cooling coils of air conditioning system has the inherent danger of possible leak of the gas into the air stream that is induced in to public areas.

Since cooling derived from liquid CO2 is predominantly due to sensible heating (without phase change) a very large volumetric flow is required. This is also associated with the need for large volume liquid CO2 receivers at pressures in excess of 25 bar. This necessitates large wall thicknesses for the receiver and consequent engineering and economic draw backs.

Separate plants for refrigeration and air conditioning duplicate the control and electrical components. Further, independent controls do not allow fine tuning of matching interfaces resulting in high energy consumption for current refrigeration.

The cascade plant controls are somewhat ad-hoc resulting in hunting and unstable operation under dynamic conditions experienced in a supermarket environment, such as refrigerated cabinets are being opened and closed by shoppers.

SUMMARY OF THE PRESENT INVENTION

In a first preferred embodiment of the invention a climate system is provided for an enclosure (eg supermarket), the enclosure having at least one sub-enclosure (eg refrigerated cabinets holding food products), the system comprising:

(a) a refrigeration plant for maintaining a first sub-enclosure within at least one predetermined temperature range including,

-   -   (i) a CO2 circuit for cooling the at least one sub-enclosure;         and     -   (ii) a non CO2 refrigerant circuit in heat exchange relationship         with the CO2 circuit;     -   and

(b) an air conditioning plant for maintaining the enclosure, external of any sub enclosure, within at least one predetermined temperature range including at least one non refrigerant fluid (eg water) circuit in heat exchange with (i) the non CO2 refrigerant circuit and (ii) an air circulation circuit for the enclosure.

Typically, there may be differing types of sub-enclosures including medium temperature and low temperature sub-enclosures. In the supermarket example these may be low temperature cabinets such as those in which frozen food is stored and accessed by a closable sealed door. In such an example there may also be medium temperature cabinets of the type typically use for displaying unfrozen meat and dairy products.

From the above examples, it will be understood that when the terms “enclosure” or “sub-enclosure” are used in this specification they include partially as well as completed enclosed configurations.

As indicated above, the non refrigerant circuit may be water. It may also be more than one circuit. For example, the water circulating may be chilled in heat exchange with the non CO2 refrigerant circuit and then pass for heat exchange to cool air in the air circulation circuit. Likewise there may be a water circuit which may be heated in heat exchange with the non CO2 refrigerant circuit and then pass for heat exchange to heat air in the air circulation circuit. In a supermarket example, this would be provide cooling and heating to the supermarket atmosphere.

In another preferred embodiment of the invention, the systems further includes a control system for optimising the operation of the system. This may provide a dynamic response of the interfacing equipment between CO2 circuit and the non CO2 refrigerant circuit segment. For example the heat exchangers and the CO2 liquid receiver are made responsive to transient conditions of operation through a judicious programming of the control system. More preferably, this control system may account for the gradients in thermodynamic properties, such as vapour pressure, specific volume and heats of vaporization, of the two refrigerants.

More preferably, the control system may also integrate the operation of air conditioning and refrigeration systems to provide optimum operating conditions of maintaining store temperature and relative humidity vis-à-vis infiltration load on the cooling coils of medium temperature enclosures.

The following highlights some specific technical advantages which may be achieved by the integrated approach of the invention.

According to another preferred aspect of this invention, the cascading temperature between the CO2 circuit and the non CO2 refrigerant circuit is −7° C. or higher which despite raising the CO2 system pressure, enables operation of the non CO2 refrigerant circuit at a lowered suction specific volume resulting in being able to use compact reciprocating compressors.

A corollary aspect of this is increased volumetric and isentropic efficiencies because of smaller pressure ratios across the non CO2 refrigerant circuit compressors.

According to another preferred aspect, partial evaporation of CO2 is allowed in the coils of the medium temperature sub-enclosure which tremendously increases the heat transfer coefficients inside the cooling coils that would make it suffice to use smaller number of pipes in the coil. A consequential benefit of smaller number of pipes in the coil is a reduced pressure drop of air and hence diminished fan power requirements, given that the coil is the largest source of pressure drop in the air stream.

Another auxiliary aspect of the partial evaporation of CO2 in the coils of the medium temperature sub-enclosure is that the required amount of storage of liquid CO2 is considerably reduced. This will result in a smaller volume of the CO2 liquid receiver which will offset the increased operating pressure arising out of raising the cascade temperature.

An ensuing aspect of partial evaporation is that no liquid sub-cooling is required for CO2 while recirculating in the coils of the medium temperature sub-enclosure. This has been the spin off of raising cascade temperature.

According to another preferred aspect, the non CO2 refrigerant circuit integrates the actions of condensation of CO2 vapour arriving from the low temperature CO2 compressor, condensation of partially evaporated CO2 in coils of the medium temperature sub-enclosure and chilling the water for air conditioning needs.

According to yet another preferred aspect, the heat rejection by the non CO2 refrigerant circuit after compression, occurs in a compact heat exchanger that drastically reduces the amount of non CO2 refrigerant in the circuit.

More preferably condensation heat is recovered to some extent via the use of compact heat exchanger by raising the temperature of water which is used for heating purposes in the air conditioning segment.

A further aspect of use of a compact water cooled heat exchanger and heat recovery is that the size of the heat rejection coil that cools the water back to near ambient conditions will be more efficient than envisaged in the current-state-of-the-art.

Preferably the liquid non CO2 refrigerant entering one of the cascade heat exchangers is sub-cooled using the CO2 vapour returning from the low temperature direct expansion segment. More preferably, sub-cooling is applied to both the streams of HFC liquid that enter the two cascading heat exchangers. Even more preferably there can be a plurality heat exchangers and a plurality of CO2 liquid receivers.

Preferably, non CO2 refrigerant circuit operates with HFC refrigerants such as R404a, 507a, 134a, 407c, 410a that are currently in vogue or more preferably replaced with any other refrigerant, natural (such as ammonia or hydrocarbons) or synthetic (such as now emerging HFO 1234yf) that might emerge in future as replacements for preferred refrigerants.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will now be further illustrated with reference to the non limiting example depicted in the accompanying figure.

The low temperature segment (designated by 10 series of numbered components) operated in a closed cycle conventional vapour compressor refrigeration will have CO2 as the refrigerant and use a single or plurality of positive displacement type of compression system (10). The discharge gas of this compressor is directly led into the vapour line of CO2 entering the cascade heat exchangers (200 and 201). This vapour line not only receives the vapour from these compressors (13A and 13B), but also from the vapour space of the CO2 receiver (14A and 14B). An important requirement is that the CO2 liquid receiver (16) pressure (p16) must be matched with the discharge pressure (p13) of the compressor. Notionally, this pressure will be the saturation pressure of CO2 at the cascading temperature which is typically set at −7° C. in the description without prejudice to any other temperature that can be adopted and all such variations are included.

Those familiar with the art of refrigeration would appreciate that the temperatures of the two streams, namely that arriving from 14A and 14B from the receiver (16) and 13A and 13B from the compressor (10) will be different and the mixture temperature that actually enters the compact heat exchangers (200 and 201) will depend on the mass flow rates of each of these streams. For example, when the load on the freezers (600 A to N) is small and only some of the compressors (10) are operating, the temperature of the stream will be the discharge temperature of the gas from the compressors and is solely governed by the design of a specific make of compressor that is used in any particular application. Qualified professionals also appreciate that the pressures (p13's) are also variable depending on the balance point of operation at any instant. It must also be appreciated that all these values are entirely transient in nature. The change in the pressure (p13 and p16) will also lead to change in temperature of the two streams. This is due to thermodynamic saturation vapour pressure of CO2 in the receiver and the discharge temperature of the gas coming from the compressor. One important aspect of this design is the splitting of flows into two identically sized heat exchangers (200 and 201). The warm vapour of CO2 is liquefied in the heat exchangers and then drains down to the liquid receiver (15A and 15B) by gravity.

Cooling of CO2 in the heat exchangers (200 and 201) is provided by evaporating high stage refrigerant. This refrigerant, HFC 134a for the sake of further description, can be any other refrigerant including CO2. Synthetic refrigerants such as 404a, 507a, 407c or 410a or natural refrigerants such as ammonia or hydrocarbons, and any other refrigerants being proposed (eg HFO 1234yf) or expected to be discovered in the future for such duties are all deemed to be inclusive in the claims of this patent.

HFC 134a evaporates at a temperature lower than the condensing temperature of CO2 (T122A and T122B <T15A and T15B) and this difference is governed by the manifestation of the flow rates of the HFC134a and CO2 streams. Typically a minimum difference of 2° C. will be required and those well versed in the art would appreciate that operation at differences larger than about 8° C. would lead to entropy generation although there is no bar on such an operation.

The cooling stream of HFC 134a is in a closed circuit (designated by 100 series of components) which again operates on a vapour compression refrigeration cycle as described in the thermodynamic concepts. The present state-of-the-art is to use positive displacement compressors (100) (such as reciprocating, rotary, scroll or screw type) and the present invention does not exclude the use of centrifugal compressors.

The HFC 134a cooling circuit provides two types of refrigeration. Firstly, it enables condensation of CO2 in the heat exchangers (200 and 201) and secondly it provides cooling for chilled water circuit in another heat exchanger (202). Those skilled in the art will appreciate that the chilled water heat exchanger cannot be operated below the freezing point of water (−0° C.), where as the CO2 circuit must be operated well below this temperature. The design intelligence of the controls addresses both these cooling requirements although the same plurality of compressors is used for the entire HFC 134a refrigeration circuit. A set of evaporator pressure regulators (125) are used to achieve this discrimination in the evaporating temperatures in 202.

As depicted only one of the liquid streams (116A) of HFC 134a is sub-cooled using the return vapour of CO2 from the low temperature direct expansion cases in the heat exchanger (204). The objective is to increase the temperature of CO2 (T23) to obviate the ductile to brittle transition of the compressor body made of cast materials. At the same time this enables sub-cooling of HFC 134a liquid (116A) at a nominal condensing temperature (typically, in the range of 40-45° C.) by a few ° C. (117A). Only one stream is sub-cooled to control this level of superheat easily, although the present innovation does not preclude sub-cooling both streams (114 and 116A) of liquid HFC 134a entering the two cascade heat exchangers (200 and 201).

In applications involving a large number of low temperature direct expansion (DX) systems (such as in biological specimen preservation) more than one liquid CO2 receivers can be used. In addition it is possible that the low temperature DX systems are directly supplied with CO2 without a pump. All such variations are part and parcel of the embodiments.

Condensation of HFC 134a occurs in another heat exchanger (205) which is water cooled. Water providing cooling of hot vapour of HFC 134a is again in a closed circuit (designated by 400 series of components). The warm water produced in the process (404) is used in the air handling unit of the air conditioning system. This unit has two coils, namely, one circulating chilled water (207) produced from (202) and another for warm water in 208. In effect, the major portion of the waste heat of condensation of HFC 134a is recovered and gainfully used. This embodiment was not possible in known configurations where the air conditioning and refrigeration were provided by two separate plants and controlled independently.

Inter-Relationship Between Refrigeration and Air Conditioning

The operation of the refrigeration plant (which was meant for cooling at or below the freezing point of water (such as preservation of food)) was intricately related to the envelope in which they are located. For example, in a supermarket, the display cases are expected to be located in an air conditioned environment. Nearly 70% cooling load on an open fronted medium temperature display cases (800 A to N) is due to infiltration of humid ambient air into the air curtain.

One way of managing the refrigeration load of the display cases is to manage the absolute humidity of the ambient air. A typical condition for which the display cases are rated (from which the refrigeration plant load is calculated) is a temperature of 25° C. and a relative humidity of 60% yielding an absolute humidity of 12 g/kg of dry air for a normal atmospheric pressure condition. The humidity level for a −5° C. air exit temperature at the coil will be about 2.5 g/kg of dry air. The cooling coil will frost up at the rate of about 9.5 g/kg of air handled by it in the display case. This is one of the reasons for perturbations (variances) in the load on the refrigeration plant. The numerical values used herein are only for the sake of an example and this invention is deemed to include other values that are relevant a particular condition of usage.

When the external ambient conditions change such as in summer, the refrigeration plant serving the low and medium temperature cases struggles to meet the load because of higher condensing temperatures and reduced volumetric efficiencies of the compressor bank (100).

One way to reduce the load on the plant is to lower the absolute humidity of the store condition to say 9 g/kg of dry air by achieving higher dehumidification in the cooling coil. This can be done by lowering the dew point temperature at the coil from about 10 to 5° C. This can be done easily because the HFC compressor bank (100) that services the chilled water to the cooling coil (207) in the air handling unit is the same as the one that services the condensation of CO2 and is already operating at a suction temperature lower than that necessary for the reduction of dew point temperature. Thus, this embodiment allows actually reducing the load on the refrigeration plant during adverse operating conditions through an intelligent linking of operating conditions of air conditioning and refrigeration circuits.

Most refrigeration plants operate under a constant head pressure, for example p13 in the CO2 circuit and p104 in HFC circuit. The regulation of p13 is achieved by intelligent operation of heat exchangers 200 and 201 though manipulation of flows and pressures in the HFC circuit. Regulation of p104 is done by yet another intelligent operation of heat exchanger 205 and manipulation of flow rates in the water line 403 made possible by the water pump 401. The heat acquired by water circuit in 205 is rejected to the ambient in the dry cooler 206.

The dry cooler is a heat exchanger transferring heat from warm water to air being drawn by a set of fans 700A and 700B.

Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this invention.

The word ‘comprising’ and forms of the word ‘comprising’ as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions.

In this specification, including the background section, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or known to be relevant to an attempt to solve any problem with which this specification is concerned. 

1. A climate system for an enclosure, the enclosure having at least one sub-enclosure, the system comprising: (a) a refrigeration plant for maintaining a first sub-enclosure within at least one predetermined temperature range including, (i) a CO2 circuit for cooling the at least one sub-enclosure; and (ii) a non CO2 refrigerant circuit in heat exchange relationship with the CO2 circuit; and (b) an air conditioning plant for maintaining the enclosure, external of any sub enclosure, within at least one predetermined temperature range including at least one non refrigerant fluid circuit in heat exchange with (i) the non CO2 refrigerant circuit and (ii) an air circulation circuit for the enclosure.
 2. The system of claim 1 wherein between the CO2 circuit and the non CO2 refrigerant circuit a cascading temperature of −7° C. or higher is maintained.
 3. The system of claim 1, wherein the sub-enclosure(s) are medium temperature and/or low temperature sub-enclosures.
 4. The system of claim 1, comprising a first and a second non refrigerant fluid circuit, the first circuit for circulating chilled fluid for heat exchange with the air circulation circuit to cool air and the second circuit for circulating warm fluid for heat exchange with the air circulation circuit to warm air.
 5. The system of claim 1 further comprising a control system for optimising the operation of the system.
 6. The system claim 4 wherein the control system includes one or more sensors to sense perturbations in the operation of the system and one or more transmitters to generate signals to the CO2 circuit and the non CO2 refrigerant circuit in response to sensing said perturbations. 